My Sodastream CO2 system runs 270g bottles which cost $12 to exchange at Mitre10 Mega or Farmers about once every 3-4weeks. Compare that to the Fluval cartridges which contain either 20g of CO2 for $12 each or 88g of CO2 for $35.60 (going by current price at HFF). Now my tank is 100L whereas yours is 23L so you should go through about 75% less CO2 than I do.
So here's the numbers for the Fluval system:
$200 for the 88g kit which comes with one 88g canister.
To replace this canister it'll cost you $35.60.
By my calculations, you'll use about 67.5g in the same time I use 270g so you'll be paying $35.60 roughly once a month.
And here's the numbers for a soda stream system:
$78.94 + shipping for a regulator.
$40ish for an adaptor.
$20 for the glassware from the same site as regulator.
$20 for a bottle off TM or buy one new (unsure of price) from Mitre10 Mega.
That's running up $160 with refills costing $12 once every 3 months.
Up to you though, the Fluval will be easier to set up initially but the soda stream setup will cost you a lot less to set up and run.